Conventionally, in general, a coextruded polypropylene-based resin laminate film wherein a polyolefin-based resin having a low melting point is laminated on a polypropylene-based resin, and a polypropylene-based resin laminate film, wherein an unoriented polyethylene-based resin film or polypropylene-based resin film and an oriented polypropylene-based resin film are laminated, have been frequently used as heat-sealable films used for packaging. However, a coextruded polypropylene-based resin laminate film wherein a polyolefin-based resin having a low melting point is laminated on a polypropylene-based resin has a certain level of seal strength but does not have a seal strength enabling packaging of a heavy article such as a water product and the like. A polypropylene-based resin laminate film, wherein an unoriented polyethylene-based resin film or polypropylene-based resin film and an oriented polypropylene-based resin film are laminated, has sufficient seal strength but requires a laminating step using an organic solvent and the like, and therefore, is unpreferable in view of the economical aspect and an influence on the global environment.
In addition, as a method for improving the seal strength of the coextruded polypropylene-based resin laminate film, an embodiment using a polypropylene-based resin as a substrate layer and a linear low density polyethylene-based resin as a heat-sealable layer has been reported (e.g., see patent reference 1). However, since a heterologous material is laminated, it has a weak interlayer strength, and a sufficient heat seal strength desired by the present invention cannot be obtained.    patent reference 1: JP-A-9-207294
Moreover, forming an adhesion layer between a polypropylene-based resin layer and a linear low-density polyethylene-based resin layer has been reported (e.g., see patent reference 2). However, since a polyethylene-based resin is laminated, the film loses elasticity, which causes difficulty in the handling during bag making processing and after filling the contents.    patent reference 2: JP-A-10-76618
In addition, it has been reported that sufficient seal strength can be obtained by defining the MFR (melt flow rate) of the resins to be used for the respective layers (e.g., see patent reference 3). However, this alone is not sufficient to ensure satisfactory tight-sealing property of the sealed part during automatic packaging, and this method is unsuitable for use requiring tight-sealing property of the sealed part.    patent reference 3: JP-A-2003-225979